Long Range Molecular Vibration Correlation in Liquids

在这个由化学系化学结构、动力学和机制A（CSDM-A）项目资助的项目中，内华达州拉斯维加斯大学的大卫谢尔顿教授正在使用非弹性激光散射技术来探测液体中的分子振动。 中午天空的蓝色和黎明或黄昏的红色是光的弹性散射的结果;光的光子保持相同的颜色（即，能量），因为它们在大气中散射分子或尘埃颗粒。 有时，光的光子会将部分能量损失给它们散射的分子或物体。 这被称为非弹性散射，对散射光子能量的分析可以揭示关于散射系统的有趣信息。 谢尔顿教授正在研究液体中分子的振动。 普遍接受的观点是，液体中的分子振动是局部的，一个振动分子对另一个分子运动的影响最多延伸几个分子直径。 该项目正在探索分子之间的振动相互作用可以发生在数百个分子直径的分子间距离的可能性（基于一些初步观察）。 如果这些初步观察得到证实，我们对液体的理解将发生重大变化：尽管某些液体显然是由随机移动的单个分子组成的，但它们似乎具有一些通常与高度有序的固体晶体相关的性质。 该项目中使用的主要工具（超拉曼散射光谱）是一种先进的非弹性散射仪器，当两个散射光子联合收割机结合形成一个高能光子时（这是所谓的“非线性”光学过程的一个例子），它可以检测事件。本项目的研究对象是大学本科生，他们在先进的非线性激光散射技术和光谱分析方面获得了非常宝贵的经验，这两项技术在其他科学领域都很重要，在传感和通信技术中也越来越重要。本项目的重点是通过超拉曼散射（HRS）测量几个代表性的非极性、非极性和非极性分子的长程分子振动相关，极性和氢键分子液体。HRS用入射在样品上的线性偏振激光束和以90度散射角收集的线性偏振散射光观察。每个振动模式的HRS出现在从激光器的第二谐波频率偏移模式的振动频率的散射光频率处。对于IS = VV、HV、VH和HH的配置，用平行（H）或垂直（V）于水平散射平面偏振的入射（I）和散射（S）光测量光谱强度。HV和VH HRS的强度是相等的对称性为不相关的分子振动在各向同性流体中，但当分子振动空间相关，然后相干添加从单个分子的散射场产生的依赖性的散射波矢量，打破了HV和VH的偏振配置之间的对称性。HRS信号被分解为横向和纵向极性集体模式的贡献，基于其独特的偏振（和角度）的依赖性。这个实验表明，液体中的分子振动在宏观距离上是相关的，这是通常描述液体的一个根本变化。这项工作有助于更好地理解物质的液态，这对技术，物理，化学和生物学都很重要。 超拉曼散射实验的设计是为了让没有丰富经验的本科生研究人员也能使用;新的经验为他们进入研究生院和其他高级研究做好了准备。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor David Shelton of the University of Nevada Las Vegas is using inelastic laser light scattering techniques to probe molecular vibrations in liquids. The blue color of the sky at mid-day and red color at dawn or dusk are the result of elastic scattering of light; the photons of light maintain the same color (i.e., energy) as they scatter off molecules or dust particles in the atmosphere. Sometimes, the photons of light lose some of their energy to the molecule or object from which they scatter. This is called inelastic scattering, and analysis of the scattered photon energy can reveal interesting information about the scattering system. Prof. Shelton is exploring the vibrations of molecules in liquids. The generally accepted view is that molecular vibrations in liquids are localized, with the influence of one vibrating molecule on the motion of another molecule extending at most a few molecular diameters. This project is exploring the possibility (based on some preliminary observations) that vibrational interactions between molecules can occur at inter-molecular distances of hundreds of molecular diameters. If these preliminary observations are confirmed, our understanding of liquids would be changed significantly: despite being composed of apparently randomly moving individual molecules, some liquids would appear to possess some properties normally associated with highly ordered solid crystals. The principal tool used in this project (hyper-Raman scattering spectroscopy) is an advanced type of inelastic scattering instrument that detects events when two scattering photons combine to form one high-energy photon (this is an example of what is called a “nonlinear” optical process). The students engaged in this project are undergraduate researchers who are gaining an unusually valuable experience in advanced nonlinear laser light scattering techniques and spectroscopic analysis, both of which are important in other areas of science and increasingly important in sensing and communications technologies.The project focuses on long range molecular vibration correlation measured by hyper-Raman scattering (HRS) in several representative non-polar, polar and hydrogen-bonding molecular liquids. HRS is observed with a linear polarized laser beam incident on the sample and linear polarized scattered light collected at 90 degree scattering angle. HRS for each vibration mode appears at a scattered light frequency that is shifted from the second harmonic frequency of the laser by the vibration frequency of the mode. Spectral intensity is measured with incident (I) and scattered (S) light polarized either parallel (H) or perpendicular (V) to the horizontal scattering plane, for the configurations with IS = VV, HV, VH and HH. The HV and VH HRS intensities are equal by symmetry for uncorrelated molecular vibrations in an isotropic fluid, but when the molecular vibrations are spatially correlated, then coherent addition of the scattered fields from individual molecules produces a dependence on the scattering wavevector that breaks the symmetry between the HV and VH polarization configurations. The HRS signal is decomposed into contributions from transverse and longitudinal polar collective modes based on their distinctive polarization (and angle) dependence. This experiment suggests that molecular vibrations in liquids are correlated over macroscopic distances, which is a fundamental change in the usual description of liquids. This work contributes to a better understanding of the liquid state of matter, which is important for technology, physics, chemistry and biology. The hyper-Raman scattering experiments have been designed to be accessible to undergraduates researchers without extensive prior experience; the new experience is preparing them well for graduate school and other advanced pursuits.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.